A device for detecting the acceleration of a vehicle is disclosed for example, in Tokkai Hei 8-15312 published by the Japanese Patent Office in 1996. In this device, a forward/backward acceleration of the vehicle is found based on the period of a pulse signal emitted in a given interval by a vehicle speed sensor. This device measures the time required to count the past n pulse signals. A difference between this required time and the time required to count the preceding n pulse signals measured in the same way is calculated, and the acceleration of the vehicle is found by multiplying this difference by a predetermined coefficient. n may for example be set equal to the number of pulses generated per one rotation of a pulse generating rotor of the vehicle speed sensor in one minute.
Since the deceleration of the vehicle is expressed as negative acceleration, it should be noted that the term acceleration in the following expression involves deceleration.
In the above device, the pulse signal is a rectangular wave, and the acceleration is calculated by performing the above measurement each time the edge of the pulse signal, i.e. the appearance of the signal, is detected. This calculation must be performed before the next pulse signal is output. Since this calculation contains a multiplication that takes time, a microprocessor of high performance must be used so that the calculation is performed even at high vehicle travel speeds when the pulse signal interval is short.
The vehicle speed sensor for example comprises a digital rotation sensor. This comprises a pulse generation rotor comprising a gear with teeth or a disc with holes rotating at a speed in proportion to the vehicle speed, and an optoelectric element that generates a pulse signal every time when a tooth or hole passes by. Such a sensor is known from Tokkai Hei 8-15312 published by the Japanese Patent Office in 1996.
In such a vehicle speed sensor, the number of pulse signals increases at high vehicle speeds and the load on a microprocessor performing the acceleration calculation increases. When the vehicle speed is extremely high, the time required for calculation exceeds the interval of the pulse signal, and the correct detection of acceleration becomes difficult. A high performance microprocessor must therefore be used.
In a vehicle using a torque converter with a lockup clutch, there is a possibility that the engine may stall when the vehicle suddenly decelerates while the lockup clutch is engaged. Therefore, the lockup clutch must be released when for example, the deceleration of the vehicle reaches a predetermined value so as to prevent this engine stall.
In this case, the data necessary for operation of the lockup clutch concern whether or not the deceleration of the vehicle reached the predetermined value, the actual value of the deceleration itself being unnecessary.
However, when the vehicle speed sensor of the aforesaid prior art is used for the above-mentioned determination, first, the value of the deceleration is calculated, and is then compared with the predetermined value. Therefore even in an acceleration detector applied to such a use, a high performance microprocessor is required to obtain a precise result when the vehicle speed is high.
On the other hand, engine stall generally occurs during rapid deceleration from a low rotation speed, and does not easily occur during rapid deceleration at high rotation speed. It is therefore unnecessary to release lockup to prevent engine stall in the high vehicle speed region. In other words, even if a high performance microprocessor is used for the detection of acceleration, when the vehicle speed is high.
Also, when a vehicle runs on an uneven road, the microprocessor may determine that a set deceleration for releasing lockup was achieved. However, the variation of the vehicle speed due to unevenness of the road surface is actually only a noise component and therefore it should not be taken into consideration to determine whether or not the lockup clutch is released.